The present invention relates to a mixture which can be polymerized by radiation, particularly by light, and which contains as the essential constituents, a binder which is insoluble in water but soluble or swellable in aqueous alkaline solutions, a photoinitiator and a polyglycol dimethacrylate or diacrylate.
The mixture of the invention is particularly suitable as a photoresist for production of printed circuits, electroplating resists and etched microstructures. Further fields of application include production of etched gravure printing forms and relief printing forms.
Negative-acting photoresist lacquers which are applied in the form of a solution to the supports to be treated, are based essentially on the following types of compounds: polyvinyl cinnamates (U.S. Pat. No. 2,690,966 and U.S. Pat. No. 2,732,301), polyvinyl cinnamylidene esters (U.S. Pat. No. 3,549,368), polystyrene cinnamyl ketone (U.S. Pat. No. 2,566,302) and other polymers containing cinnamoyl groups (German Pat. Nos. 1,063,802 and 1,067,219 and German Offenlegungsschrift No. 2,111,415), allyl ester prepolymers (U.S. Pat. Nos. 3,376,138; 3,376,139 and 3,462,267) and polyisoprenes (U.S. Pat. No. 2,852,379 and British Pat. No. 892,811).
Although some of these substances are used industrially as photoresist solutions, most of them have certain disadvantages. They require organic solvents for development, or they have a relatively low sensitivity to light. Others have a poor adhesion to metal surfaces, or can only be processed to give relatively small layer thickness.
Negative-acting liquid photoresist compositions which depend on crosslinking by photoinitiated free-radical polymerization and which can be developed in aqueous alkaline solutions, are described, for example, in British Pat. No. 1,377,512. However, the mixtures described in that document are sensitive to oxygen. For this reason, the layers must be provided with a polyvinyl alcohol covering layer.
A further considerable problem is tackiness of photopolymerizable layers, which can make contact exposure impossible. Thus the layers described in British Pat. No. 1,379,228 exhibit a tendency toward adhesion or imprinting if they are exposed in direct contact with an original.
British Pat. No. 2,059,982 describes photopolymerizable mixtures which can advantageously be used as pre-formed, transferable photoresist layers applied to a plastic film as a temporary support. These layers are transferred to the final support by lamination under heat and pressure. For this purpose, they contain polymerizable compounds of a fairly high molecular weight, namely diacrylates or dimethacrylates of polyglycols, especially polyethylene glycols having 14 to 23 glycol units, in order to prevent evaporation of the monomer at the laminating temperature.
The use of this type of mixture as a photoresist solution, which is applied as such by the user directly to the support to be treated and is dried, has not heretofore gained acceptance in the art because it is too difficult for the user to apply the protective covering layers or films to the photoresist layer.